Before deciding which system you wish to use you may want to consider the differences: Roasting over open coals can take twice as long as it does inside a propane roaster and you will need to be on hand to top up your fire with pre-heated coals ever hour to ensure that you get even cooking and crispy skin.
The general rule of thumb on a BBQ pig is 40-50% meat yield from raw butchered weight. So a 60lb Pig will give you approximately 25-30lb of cooked meat and will feed around 55 people. You’ll need at least 1lb raw weight per person for a mixed group, for a bunch of hungry men I would recommend allowing 1.5lb raw weight per person. If you are cooking over direct heat you may lose the belly meat and as such may need a larger pig. However, if you cook using a rotisserie you will get a better yield and less meat will be lost.